The present invention relates, generally, to the configuration of the surface topography of pads used in processing workpieces and, more particularly, to the use of microreplicated structures as a pad surface topography.
Chemical mechanical planarization (xe2x80x9cCMPxe2x80x9d) is widely used in the microelectronics industry, particularly for local and global planarization of VLSI devices with sub-micron geometries. A typical CMP process involves polishing back built-up layers of dielectrics and conductors on integrated circuit chips during manufacture.
More particularly, a resinous polishing pad having a cellular structure is traditionally employed in conjunction with a slurry, for example a water-based slurry comprising colloidal silica particles. When pressure is applied between the polishing pad and the workpiece (e.g., silicon wafer) being polished, mechanical stresses are concentrated on the exposed edges of the adjoining cells in the cellular pad. Abrasive particles within the slurry concentrated on these edges tend to create zones of localized stress at the workpiece in the vicinity of the exposed edges of the polishing pad. This localized pressure creates mechanical strain on the chemical bonds comprising the surface being polished, rendering the chemical bonds more susceptible to chemical attack or corrosion (e.g., stress corrosion). Consequently, microscopic regions are removed from the surface being polished, enhancing planarity of the polished surface. See, for example, Arai, et al., U.S. Pat. No. 5,099,614, issued March, 1992; Karlsrud, U.S. Pat. No. 5,498,196, issued March, 1996; Arai, et al., U.S. Pat. No. 4, 805,348, issued February, 1989; Karlsrud et aL, U.S. Pat. No. 5,329,732, issued July, 1994; and Karlsrud et aL, U.S. Pat. No. 5,498,199, issued March, 1996, for further discussion of presently known lapping and planarization techniques. By this reference, the entire disclosures of the foregoing patents are hereby incorporated herein.
Presently known polishing techniques are unsatisfactory in several regards. For example, as the size of microelectronic structures used in integrated circuits decreases to sub-half-micron levels, and as the number of microelectronic structures on current and future generation integrated circuits increases, the degree of planarity required increases dramatically. The high degree of accuracy of current lithographic techniques for smaller devices requires increasingly flatter surfaces. Presently known polishing techniques are believed to be inadequate to produce the degree of local planarity and global uniformity across the relatively large surfaces of silicon wafers used in integrated circuits, particularly for future generations.
Presently known polishing techniques are also unsatisfactory in that processes designed to produce planar, defect-free surfaces are necessarily time-consumingxe2x80x94involving extremely fine slurry particles in conjunction with porous pads.
Presently known polishing techniques are also unsatisfactory in that traditional polishing pads require periodic conditioning to maintain their effectiveness. As a result, batch-to-batch variations persist, and other complications of the conditioning step arise (for example, degradation of the conditioning pad itself).
Microreplicated structures are generally well known in other fields, particularly in the field of optics, wherexe2x80x94as a result of their retroreflective propertiesxe2x80x94microreplicated films have found wide application for use in Fresnel lenses, road signs and reflectors. In addition, larger examples of such structures (on the order of 100 microns in height) have been incorporated into structured abrasive articles useful for grinding steel and other metals (see, e.g., Pieper et al., U.S. Pat. No. 5,304,223, issued Apr. 19, 1994).
In the context of chemical-mechanical planarization, regular arrays of structures (e.g., hemispheres, cubes, cylinders, and hexagons) have been formed in standard polyurethane polishing pads (see e.g. , Yu et al., U.S. Pat. No. 5,441,598, issued Aug. 15, 1995). Such structures are typically over 250 microns in height, andxe2x80x94due to their porosityxe2x80x94suffer from the same asperity variations found in other polyurethane pads.
Chemical mechanical planarization techniques and materials are thus needed which will permit a higher degree of planarization and uniformity of that planarization over the entire surface of integrated circuit structures. At the same time, more efficient techniques are needed to increase the throughput of wafers through the CMP system while reducing batch-to-batch variation.
In accordance with a preferred embodiment of the present invention, a chemical mechanical planarization process employs a microreplicated surface or pad in lieu of the traditional cellular polishing pad employed in presently known CMP processes. For example, a microreplicated surface useful in the context of the present invention suitably consists of a regular array of precisely shaped three-dimensional structures (for example, pyramids), each of which preferably have sharp distal points. The uniformity of such a microreplicated surface provides enhanced global and local planarization. Such microreplicated pads further provide improved processing of other types of workpieces, including magnetic media, magnetoresistive (MR) heads, texturizing of pre and post-media disks, and polishing of glass and metallic media. These pads further provide a technique for planarizing workpieces with photoresist build-up along their perimeters.
In a preferred embodiment, wherein slurry particles are substantially smaller than the microreplicated structure size, chemical mechanical polishing takes place in two phases. Early on in the process, when the microreplicated surface is fresh and its asperities are relatively sharp, material removal at the workpiece surface is effected primarily through mechanical abrasion between the workpiece and the microreplicated structures. During this phase, abrasive particles in the slurry have little effect on material removal rate. As processing progresses, however, and ablation of the microreplicated polishing surface proceeds, the individual microreplicated structures become dulled. As dulling of the microreplicated structures continues, the chemical-mechanical effects of the abrasive particles become more pronounced. In view of the transitional nature of this process, a microreplicated surface is advantageously employed in a linear belt configuration, wherein the belt moves either continuously or, in a particularly preferred embodiment, advances linearly at the beginning of the process (at the completion of the previous batch of workpieces) in order to provide a fresh microreplicated surface. This ensures repeatable polishing conditions, and reduces batch-to-batch variation.
In accordance with a further aspect of the present invention, the use of a microreplicated pad in a consolidated two-phase process increases workpiece throughput by providing a high initial removal rate at the beginning of the polishing operation (when the microreplicated structures are sharp), followed gradually by a fine polishing step (as the microreplicated structures become dull).